High temperature oxidation protection for composites

ABSTRACT

The present disclosure provides a method for coating a composite structure, comprising forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition, with a primary flow modifier and a first carrier fluid, wherein the primary flow modifier comprises at least one of cellulose or calcium silicate; applying the first slurry on a surface of the composite structure to form a base layer; and heating the composite structure to a temperature sufficient to adhere the base layer to the composite structure.

FIELD

The present disclosure relates generally to carbon-carbon composites and, more specifically, to oxidation protection systems for carbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typically designed to minimize loss of carbon material due to oxidation at operating conditions, which include temperatures as high as 900° C. (1652° F.). Phosphate-based oxidation protection systems may reduce infiltration of oxygen and oxidation catalysts into the composite structure. However, despite the use of such oxidation protection systems, significant oxidation of the carbon-carbon composites may still occur during operation of components such as, for example, aircraft braking systems.

SUMMARY

A method for coating a composite structure (e.g., a carbon-carbon composite structure) is provided comprising forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition, with a primary flow modifier and a first carrier fluid, wherein the primary flow modifier comprises at least one of cellulose or calcium silicate; applying the first slurry on a surface of the composite structure to form a base layer; and/or heating the composite structure to a temperature sufficient to adhere the base layer to the composite structure. Forming the first slurry may comprise mixing the first pre-slurry composition and the first carrier fluid prior to mixing the primary flow modifier into the first slurry.

In various embodiments, the first pre-slurry composition may further comprise a secondary flow modifier that comprises at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid). In various embodiments, the first pre-slurry composition may comprise an acid aluminum phosphate, wherein a molar ratio of aluminum to phosphoric acid is between 1 to 2 and 1 to 3.

In various embodiments, the method may further comprise forming a second slurry by combining a second pre-slurry composition with a second carrier fluid, wherein the second pre-slurry composition comprises a second phosphate glass composition; applying the second slurry to the base layer; and heating the composite structure to a second temperature sufficient to form a sealing layer on the base layer.

In various embodiments, the method may further comprise applying at least one of a pretreating composition or a barrier coating to the composite structure prior to applying the first slurry to the composite structure. In various embodiments, the pretreating composition may comprise at least one of a phosphoric acid or an acid phosphate salt, an aluminum salt, or an additional salt, and the composite structure may be porous and the pretreating composition may penetrate a pore of the composite structure. In various embodiments, applying a pretreating composition may comprise applying a first pretreating composition to an outer surface of the composite structure, the first pretreating composition comprising aluminum oxide and water; heating the pretreating composition; and applying a second pretreating composition comprising at least one of a phosphoric acid or an acid phosphate salt and an aluminum salt on the first pretreating composition, wherein the composite structure is porous and the second pretreating composition penetrates a pore of the composite structure. In various embodiments, the barrier coating may comprise at least one of a carbide, a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or a silicon nitride.

In various embodiments, the first phosphate glass composition and/or the second phosphate glass composition is represented by the formula

a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

In various embodiments, a slurry for coating a composite structure may comprise a carrier fluid; a primary flow modifier comprising at least one of cellulose or calcium silicate; and a first pre-slurry composition comprising a first phosphate glass composition. In various embodiments, the first pre-slurry composition may further comprise a secondary flow modifier that is at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid).

In accordance with various embodiments, an article may comprise a carbon-carbon composite structure; and an oxidation protection system including a base layer disposed on an outer surface of the carbon-carbon structure, wherein the base layer may comprise a first pre-slurry composition having a first phosphate glass composition, and a primary flow modifier, wherein the primary flow modifier is at least one of cellulose or calcium silicate. In various embodiments, the first pre-slurry composition of the base layer may comprise a secondary flow modifier that is at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid). In various embodiments, the first pre-slurry composition of the base layer may comprise an acid aluminum phosphate wherein a molar ratio of aluminum to phosphoric acid is between 1 to 2 and 1 to 3. In various embodiments, the article may further comprise a sealing layer disposed on an outer surface of the base layer, wherein the sealing layer may comprise a second pre-slurry composition comprising a second phosphate glass composition. In various embodiments, the second pre-slurry composition may comprise acid aluminum phosphate and wherein the second phosphate glass composition is substantially free of boron nitride.

In various embodiments, the first phosphate glass composition or the second phosphate glass composition may be represented by the formula

a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z):

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;

G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y₁ is a number in the range from about 0.100 to about 0.950;

y₂ is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y₁+y₂+z)=1; and

x<(y₁+y₂).

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the detailed description and claims when considered in connection with the drawing figures, wherein like numerals denote like elements.

FIG. 1A illustrates a cross sectional view of an aircraft wheel braking assembly, in accordance with various embodiments;

FIG. 1B illustrates a partial side view of an aircraft wheel braking assembly, in accordance with various embodiments; and

FIGS. 2A, 2B, and 2C illustrate a method for coating a composite structure, in accordance with various embodiments; and

FIG. 3 illustrates experimental data obtained from testing various oxidation protection systems on composite structures, in accordance with various embodiments.

DETAILED DESCRIPTION

The detailed description of embodiments herein makes reference to the accompanying drawings, which show embodiments by way of illustration. While these embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that logical and mechanical changes may be made without departing from the spirit and scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not for limitation. For example, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option.

With initial reference to FIG. 1A and 1B, aircraft wheel braking assembly 10 such as may be found on an aircraft, in accordance with various embodiments is illustrated. Aircraft wheel braking assembly may, for example, comprise a bogie axle 12, a wheel 14 including a hub 16 and a wheel well 18, a web 20, a torque take-out assembly 22, one or more torque bars 24, a wheel rotational axis 26, a wheel well recess 28, an actuator 30, multiple brake rotors 32, multiple brake stators 34, a pressure plate 36, an end plate 38, a heat shield 40, multiple heat shield sections 42, multiple heat shield carriers 44, an air gap 46, multiple torque bar bolts 48, a torque bar pin 50, a wheel web hole 52, multiple heat shield fasteners 53, multiple rotor lugs 54, and multiple stator slots 56. FIG. 1B illustrates a portion of aircraft wheel braking assembly 10 as viewed into wheel well 18 and wheel well recess 28.

In various embodiments, the various components of aircraft wheel braking assembly 10 may be subjected to the application of compositions and methods for protecting the components from oxidation.

Brake disks (e.g., interleaved rotors 32 and stators 34) are disposed in wheel well recess 28 of wheel well 18. Rotors 32 are secured to torque bars 24 for rotation with wheel 14, while stators 34 are engaged with torque take-out assembly 22. At least one actuator 30 is operable to compress interleaved rotors 32 and stators 34 for stopping the aircraft. In this example, actuator 30 is shown as a hydraulically actuated piston, but many types of actuators are suitable, such as an electromechanical actuator. Pressure plate 36 and end plate 38 are disposed at opposite ends of the interleaved rotors 32 and stators 34. Rotors 32 and stators 34 can comprise any material suitable for friction disks, including ceramics or carbon materials, such as a carbon/carbon composite.

Through compression of interleaved rotors 32 and stators 34 between pressure plates 36 and end plate 38, the resulting frictional contact slows rotation of wheel 14. Torque take-out assembly 22 is secured to a stationary portion of the landing gear truck such as a bogie beam or other landing gear strut, such that torque take-out assembly 22 and stators 34 are prevented from rotating during braking of the aircraft.

Carbon-carbon composites (also referred to herein as composite structures, composite substrates, and carbon-carbon composite structures, interchangeably) in the friction disks may operate as a heat sink to absorb large amounts of kinetic energy converted to heat during slowing of the aircraft. Heat shield 40 may reflect thermal energy away from wheel well 18 and back toward rotors 32 and stators 34. With reference to FIG. 1A, a portion of wheel well 18 and torque bar 24 is removed to better illustrate heat shield 40 and heat shield segments 42. With reference to FIG. 1B, heat shield 40 is attached to wheel 14 and is concentric with wheel well 18. Individual heat shield sections 42 may be secured in place between wheel well 18 and rotors 32 by respective heat shield carriers 44 fixed to wheel well 18. Air gap 46 is defined annularly between heat shield segments 42 and wheel well 18.

Torque bars 24 and heat shield carriers 44 can be secured to wheel 14 using bolts or other fasteners. Torque bar bolts 48 can extend through a hole formed in a flange or other mounting surface on wheel 14. Each torque bar 24 can optionally include at least one torque bar pin 50 at an end opposite torque bar bolts 48, such that torque bar pin 50 can be received through wheel web hole 52 in web 20. Heat shield sections 42 and respective heat shield carriers 44 can then be fastened to wheel well 18 by heat shield fasteners 53.

Under the operating conditions (e.g., high temperature) of aircraft wheel braking assembly 10, carbon-carbon composites may be prone to material loss from oxidation of the carbon. For example, various carbon-carbon composite components of aircraft wheel braking assembly 10 may experience both catalytic oxidation and inherent thermal oxidation caused by heating the composite during operation. In various embodiments, composite rotors 32 and stators 34 may be heated to sufficiently high temperatures that may oxidize the carbon surfaces exposed to air. At elevated temperatures, infiltration of air and contaminants may cause internal oxidation and weakening, especially in and around brake rotor lugs 54 or stator slots 56 securing the friction disks to the respective torque bar 24 and torque take-out assembly 22. Because carbon-carbon composite components of aircraft wheel braking assembly 10 may retain heat for a substantial time period after slowing the aircraft, oxygen from the ambient atmosphere may react with the carbon matrix and/or carbon fibers to accelerate material loss. Further, damage to brake components may be caused by the oxidation enlargement of cracks around fibers or enlargement of cracks in a reaction-formed porous barrier coating (e.g., a silicon-based barrier coating) applied to the carbon-carbon composite.

Elements identified in severely oxidized regions of carbon-carbon composite brake components include potassium (K) and sodium (Na). These alkali contaminants may come into contact with aircraft brakes as part of cleaning or de-icing materials. Other sources include salt deposits left from seawater or sea spray. These and other contaminants (e.g. Ca, Fe, etc.) can penetrate and leave deposits in pores of carbon-carbon composite aircraft brakes, including the substrate and any reaction-formed porous barrier coating. When such contamination occurs, the rate of carbon loss by oxidation can be increased by one to two orders of magnitude.

In various embodiments, components of aircraft wheel braking assembly 10 may reach operating temperatures in the range from about 100° C. (212° F.) up to about 900° C. (1652° F.). However, it will be recognized that the oxidation protection compositions, systems, and methods of the present disclosure may be readily adapted to many parts in this and other braking assemblies, as well as to other carbon-carbon composite structures susceptible to oxidation losses from infiltration of atmospheric oxygen and/or catalytic contaminants. An oxidation protection system applied to a composite structure may prevent or reduce oxidation of the composite structure. In various embodiments, an oxidation protection system may comprise a base layer resulting from a first slurry, a sealing layer resulting from a second slurry, and/or layers and compositions.

In various embodiments, a method for limiting an oxidation reaction in a composite structure may comprise forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition in the form of a glass frit, powder, or other suitable pulverized form, with a first carrier fluid (such as, for example, water), applying the first slurry to a composite structure, and heating the composite structure to a temperature sufficient to dry the carrier fluid and form an oxidation protection coating on the composite structure, which in various embodiments may be referred to a base layer. The first pre-slurry composition of the first slurry may comprise additives, such as, for example, ammonium hydroxide, ammonium dihydrogen phosphate, nanoplatelets (such as graphene-based nanoplatelets), among others, to improve hydrolytic stability and/or to increase the composite structure's resistance to oxidation, thereby tending to reduce mass loss of the composite structure. In various embodiments, the first slurry may comprise a primary flow modifier. The primary flow modifier may be mixed with the first pre-slurry composition after the pre-slurry composition is combined with the carrier fluid.

In various embodiments, a slurry comprising acid aluminum phosphates having an aluminum (Al) to phosphoric acid (H₃PO₄) molar ratio of 1 to 3 or less, such as an Al:H₃PO₄ molar ratio of between 1 to 2 and 1 to 3, tends to provide increased hydrolytic stability without substantially increasing composite structure mass loss. In various embodiments, a slurry comprising acid aluminum phosphates having an Al:H₃PO₄ molar ratio between 1:2 to 1:3 produces an increase in hydrolytic protection and an unexpected reduction in composite structure mass loss.

With initia reference to FIG. 2A, a method 200 for coating a composite structure in accordance with various embodiments is illustrated. Method 200 may, for example, comprise applying an oxidation protection system to non-wearing surfaces of carbon-carbon composite brake components. In various embodiments, method 200 may be used on the back face of pressure plate 36 and/or end plate 38, an inner diameter (ID) surface of stators 34 including slots 56, as well as outer diameter (OD) surfaces of rotors 32 including lugs 54. The oxidation inhibiting composition of method 200 may be applied to preselected regions of a carbon-carbon composite structure that may be otherwise susceptible to oxidation. For example, aircraft brake disks may have the oxidation inhibiting composition applied on or proximate stator slots 56 and/or rotor lugs 54.

In various embodiments, method 200 may comprise forming a first slurry (step 210) by combining a first pre-slurry composition, comprising a first phosphate glass composition in the form of a glass frit, powder, or other suitable pulverized and/or ground form, with a first carrier fluid (such as, for example, water). In various embodiments, the first slurry may comprise an acid aluminum phosphate wherein the molar ratio of Al:H₃PO₄ may be between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. The first pre-slurry composition of the first slurry may further comprise a boron nitride additive. For example, a boron nitride (such as hexagonal boron nitride) may be added such that the resulting first pre-slurry composition comprises between about 5 weight percent and about 30 weight percent of boron nitride. Further, the pre-slurry composition may comprise between about 15 weight percent and 25 weight percent of boron nitride. As used in this context only, the term “about” means plus or minus 2 weight percent. Boron nitride may be prepared for addition to the first pre-slurry composition by, for example, ultrasonically exfoliating boron nitride in dimethylformamide (DMF), a solution of DMF and water, or 2-propanol solution. In various embodiments, the boron nitride additive may comprise a boron nitride that has been prepared for addition to the first phosphate glass composition by crushing or milling (e.g., ball milling) the boron nitride. The resulting boron nitride may be combined with the first phosphate glass composition.

In various embodiments, the first slurry may comprise a primary flow modifier, such as hydrophilic bentonite, sodium metasilicate, poly(vinyl alchohol), a salt of poly(acrylic acid), poly(vinyl acetate), calcium silicate, cellulose, kaolin, and/or mullite. In various embodiments, the primary flow modifier may be added to the first slurry during the mixing of the components of the first pre-slurry composition, or after all the components of the first pre-slurry composition are combined. In various embodiments, the primary flow modifier may be added to the first slurry after all the components of the first pre-slurry composition are combined with the carrier fluid. The presence of the primary flow modifier in the first slurry may enhance the bonding or interaction of the molecules of the first slurry to form a substantially even and/or continuous distribution of the first slurry on a composite structure. For example, cellulose comprised in the first slurry surprisingly bonds and/or interacts well with boron nitride, causing the dispersion of the first slurry and the resulting base layer to be more continuous across the composite structure, such that there are few or no gaps in the first slurry and the resulting base layer, as opposed to a first slurry without cellulose. Additionally, the primary flow modifier may also prevent or reduce sagging of the first slurry after application to the composite structure.

In various embodiments, the first slurry may comprise about 2% to about 12% by weight primary flow modifier, about 4% to about 10% by weight primary flow modifier, or about 5% to about 7% by weight primary flow modifier. The term “about” as used in this context only means plus or minus 1% by weight. In various embodiments, cellulose particles used in the first slurry may have any suitable particle size. In various embodiments, a particle size of 10 micrometers (0.00004 inch) to 50 micrometers (0.0020 inch), 15 micrometers (0.0006 inch) to 40 micrometers (0.0016 inch), 15 micrometers (0.0006 inch) to 30 micrometers (0.0012 inch), or 18 micrometers (0.0007 inch) to 22 micrometers (0.0009 inch).

The first phosphate glass composition may comprise one or more alkali metal glass modifiers, one or more glass network modifiers and/or one or more additional glass formers. In various embodiments, boron oxide or a precursor may optionally be combined with the P₂O₅ mixture to form a borophosphate glass, which has improved self-healing properties at the operating temperatures typically seen in aircraft braking assemblies. In various embodiments, the phosphate glass and/or borophosphate glass may be characterized by the absence of an oxide of silicon. Further, the ratio of P₂O₅ to metal oxide in the fused glass may be in the range from about 0.25 to about 5 by weight.

Potential alkali metal glass modifiers may be selected from oxides of lithium, sodium, potassium, rubidium, cesium, and mixtures thereof. In various embodiments, the glass modifier may be an oxide of lithium, sodium, potassium, or mixtures thereof. These or other glass modifiers may function as fluxing agents. Additional glass formers can include oxides of boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof

Suitable glass network modifiers include oxides of vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof

The first phosphate glass composition may be prepared by combining the above ingredients and heating them to a fusion temperature. In various embodiments, depending on the particular combination of elements, the fusion temperature may be in the range from about 700° C. (1292° F.) to about 1500° C. (2732° F.). The resultant melt may then be cooled and pulverized and/or ground to form a glass frit or powder. In various embodiments, the first phosphate glass composition may be annealed to a rigid, friable state prior to being pulverized. Glass transition temperature (T_(g)), glass softening temperature (T_(s)) and glass melting temperature (T_(m)) may be increased by increasing refinement time and/or temperature. Before fusion, the first phosphate glass composition comprises from about 20 mol % to about 80 mol % of P₂O₅. In various embodiments, the first phosphate glass composition comprises from about 30 mol % to about 70 mol % P₂O₅, or precursor thereof In various embodiments, the first phosphate glass composition comprises from about 40 to about 60 mol % of P₂O₅.

The first phosphate glass composition may comprise from about 5 mol % to about 50 mol % of the alkali metal oxide. In various embodiments, the first phosphate glass composition comprises from about 10 mol % to about 40 mol % of the alkali metal oxide. Further, the first phosphate glass composition comprises from about 15 to about 30 mol % of the alkali metal oxide or one or more precursors thereof. In various embodiments, the first phosphate glass composition may comprise from about 0.5 mol % to about 50 mol % of one or more of the above-indicated glass formers. The first phosphate glass composition may comprise about 5 to about 20 mol % of one or more of the above-indicated glass formers. As used herein, mol % is defined as the number of moles of a constituent per the total moles of the solution.

In various embodiments, the first phosphate glass composition may comprise from about 0.5 mol % to about 40 mol % of one or more of the above-indicated glass network modifiers. The first phosphate glass composition may comprise from about 2.0 mol % to about 25 mol % of one or more of the above-indicated glass network modifiers.

In various embodiments, the first phosphate glass composition may represented by the formula:

a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z)   [1]

In Formula 1, A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂). The first phosphate glass composition may be formulated to balance the reactivity, durability and flow of the resulting glass barrier layer for optimal performance.

In various embodiments, first phosphate glass composition in glass frit form may be combined with additional components to form the first pre-slurry composition. For example, crushed first phosphate glass composition in glass frit form may be combined with ammonium hydroxide, ammonium dihydrogen phosphate, nanoplatelets (such as graphene-based nanoplatelets), among other materials and/or substances. For example, graphene nanoplatelets could be added to the first phosphate glass composition in glass frit form. In various embodiments, the additional components of the first pre-slurry composition may be combined and preprocessed before combining them with the first phosphate glass composition in glass frit form. Other suitable additional components include, for example, surfactants such as, for example, an ethoxylated low-foam wetting agent, and/or a secondary flow modifier, such as, for example, poly(vinyl alcohol), polyacrylate salt(s) or poly(acrylic acid), and/or similar polymers. In various embodiments, the addition of a secondary flow modifier may decrease the viscosity of the first slurry such that the first slurry may be more easily applied (such as in step 220, described herein) via a spraying method. In various embodiments, the first slurry may comprise between 0.1% and 2.0% by weight secondary flow modifier, between 0.3% and 1.5% by weight secondary flow modifier, or between 0.5% and 1.2% by weight secondary flow modifier.

In various embodiments, in which the secondary flow modifier is a salt of poly(acrylic acid) (e.g., sodium salt of poly(acrylic acid)), the particle size used in the first slurry may be between 1250 and 7500 grams per mole (g/mol), 1500 and 6000 g/mol, 1700 and 2500 g/mol, or 4500 and 5500 g/mol.

In various embodiments, other suitable additional components may include additives to enhance impact resistance and/or to toughen the base layer, such as, for example, at least one of whiskers, nanofibers or nanotubes consisting of nitrides, carbides, carbon, graphite, quartz, silicates, aluminosilicates, phosphates, and the like. In various embodiments, additives to enhance impact resistance and/or to toughen the barrier coating may include silicon carbide whiskers, carbon nanofibers, boron nitride nanotubes and similar materials known to those skilled in the art.

In various embodiments, method 200 further comprises applying the first slurry to a composite structure (step 220). Applying the first slurry may comprise, for example, spraying or brushing the first slurry of the first phosphate glass composition on to an outer surface of the composite structure. Any suitable manner of applying the base layer to the composite structure is within the scope of the present disclosure. As referenced herein, the composite structure may refer to a carbon-carbon composite structure.

In various embodiments, method 200 may further comprise a step 230 of heating the composite structure to form a base layer of phosphate glass. The composite structure may be heated (e.g., dried or baked) at a temperature in the range from about 200° C. (292° F.) to about 1000° C. (1832° F.). In various embodiments, the composite structure is heated to a temperature in a range from about 600° C. (1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (292° F.) to about 900° C. (1652° F.), or further, between about 400° C. (752° F.) to about 850° C. (1562° F.). Step 230 may, for example, comprise heating the composite structure for a period between about 0.5 hour and about 8 hours, wherein the term “about” in this context only means plus or minus 0.25 hours. The base layer may also be referred to as a coating.

In various embodiments, the composite structure may be heated to a first, lower temperature (for example, about 30° C. (86° F.) to about 400° C. (752° F.)) to bake or dry the base layer at a controlled depth. A second, higher temperature (for example, about 300° C. (572° F.) to about 1000° C. (1832° F.)) may then be used to form a deposit from the base layer within the pores of the composite structure. The duration of each heating step can be determined as a fraction of the overall heating time and can range from about 10% to about 50%, wherein the term “about” in this context only means plus or minus 5%. In various embodiments, the duration of the lower temperature heating step(s) can range from about 20% to about 40% of the overall heating time, wherein the term “about” in this context only means plus or minus 5%. The lower temperature step(s) may occupy a larger fraction of the overall heating time, for example, to provide relatively slow heating up to and through the first lower temperature. The exact heating profile will depend on a combination of the first temperature and desired depth of the drying portion.

Step 230 may be performed in an inert environment, such as under a blanket of inert gas or less reactive gas (e.g., nitrogen, argon, other noble gases and the like). For example, a composite structure may be pretreated or warmed prior to application of the base layer to aid in the penetration of the base layer. Step 230 may be for a period of about 2 hours at a temperature of about 600° C. (1112° F.) to about 800° C. (1472° F.), wherein the term “about” in this context only means plus or minus 10° C. The composite structure and base layer may then be dried or baked in a non-oxidizing, inert or less reactive atmosphere, e.g., noble gasses and/or nitrogen (N₂), to optimize the retention of the first pre-slurry composition of the base layer in the pores of the composite structure. This retention may, for example, be improved by heating the composite structure to about 200° C. (392° F.) and maintaining the temperature for about 1 hour before heating the carbon-carbon composite to a temperature in the range described above. The temperature rise may be controlled at a rate that removes water without boiling, and provides temperature uniformity throughout the composite structure.

In various embodiments and with reference now to FIG. 2B, method 300, which comprises steps also found in method 200, may further comprise applying at least one of a pretreating composition or a barrier coating (step 215) prior to applying the first slurry. Step 215 may, for example, comprise applying a first pretreating composition to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly 10. In various embodiments, the first pretreating composition comprises an aluminum oxide in water. For example, the aluminum oxide may comprise an additive, such as a nanoparticle dispersion of aluminum oxide (for example, NanoBYK-3600®, sold by BYK Additives & Instruments). The first pretreating composition may further comprise a surfactant or a wetting agent. The composite structure may be porous, allowing the pretreating composition to penetrate at least a portion of the pores of the composite structure.

In various embodiments, after applying the first pretreating composition, the component may be heated to remove water and fix the aluminum oxide in place. For example, the component may be heated to between about 100° C. (212° F.) and 200° C. (392° F.), and further, between 100° C. (212° F.) and 150° C. (302° F.).

Step 215 may further comprise applying a second pretreating composition. In various embodiments, the second pretreating composition may comprise an acid aluminum phosphate, which may be comprised of at least one of phosphoric acid or an acid phosphate salt, and an aluminum salt (e.g., aluminum phosphate, aluminum hydroxide, and/or aluminum oxide). The second pretreating composition may further comprise, for example, a second metal salt such as a magnesium salt. In various embodiments, the aluminum to phosphorus molar ratio of the acid aluminum phosphate may be 1 to 3 or less. Further, the second pretreating composition may also comprise a surfactant or a wetting agent. In various embodiments, the second pretreating composition is applied to the composite structure atop the first pretreating composition. The composite structure may then, for example, be heated. In various embodiments, the composite structure may be heated between about 600° C. (1112° F.) and about 800° C. (1472° F.), and further, between about 650° C. (1202° F.) and 750° C. (1382° F.).

Step 215 may further comprise applying a barrier coating to an outer surface of a composite structure, such as a component of aircraft wheel braking assembly 10. In various embodiments the barrier coating composition may comprise carbides and/or nitrides, including at least one of a boron nitride, silicon carbide, titanium carbide, boron carbide, silicon oxycarbide, and silicon nitride. In various embodiments, the barrier coating composition may comprise molybdenum disulfide and/or tungsten disulfide instead of, or in addition to, carbides and/or nitrides. In various embodiments, the barrier coating may be formed by treating the composite structure with molten silicon. The molten silicon is reactive and may form a silicon carbide barrier on the composite structure. Step 215 may comprise, for example, application of the barrier coating by spraying, chemical vapor deposition (CVD), molten application, or brushing the barrier coating composition on to the outer surface of the carbon-carbon composite structure. Any suitable manner of applying the base layer to composite structure is within the scope of the present disclosure.

In various embodiments and with reference now to FIG. 2C, method 400 may further comprise a step 240, similar to step 210, of forming a second slurry by combining a second pre-slurry composition, which may comprise a second phosphate glass composition in glass frit or powder form, with a second carrier fluid (such as, for example, water). In various embodiments, the second slurry may comprise an acid aluminum phosphate wherein the molar ratio of aluminum (Al) to phosphoric acid (H₃PO₄) may be between 1:2 to 1:3, between 1:2.2 to 1:3, between 1:2.5 to 1:3, between 1:2.7 to 1:3 or between 1:2.9 to 1:3. In various embodiments, the second slurry may comprise a second pre-slurry composition comprising acid aluminum phosphate and orthophosphoric acid with an aluminum to phosphate molar ratio of 1:2 to 1:5, and may be substantially phosphate glass free. As used herein “substantially free” means comprising less than 0.01% by weight of a substance. Further, step 240 may comprise spraying or brushing the second slurry of the second phosphate glass composition on to an outer surface of the base layer. Any suitable manner of applying the sealing layer to the base layer is within the scope of the present disclosure.

In various embodiments, the second slurry may be substantially free of boron nitride. In this case, “substantially free” means less than 0.01 percent by weight. For example, the second pre-slurry composition may comprise any of the components of the pre-slurry compositions and/or glass compositions described in connection with the first pre-slurry composition and/or first phosphate glass composition, without the addition of a boron nitride additive. In various embodiments, the second pre-slurry mixture may comprise the same pre-slurry composition and/or phosphate glass composition used to prepare the first pre-slurry composition and/or the first phosphate glass composition. In various embodiments, the second pre-slurry composition may comprise a different pre-slurry composition and/or phosphate glass composition than the first pre-slurry composition and/or first phosphate glass composition.

In various embodiments, the first slurry and/or the second slurry may comprise an additional metal salt. The cation of the additional metal salt may be multivalent. The metal may be an alkaline earth metal or a transition metal. In various embodiments, the metal may be an alkali metal. The multivalent cation may be derived from a non-metallic element such as boron. The term “metal” is used herein to include multivalent elements such as boron that are technically non-metallic. The metal of the additional metal salt may be an alkaline earth metal such as calcium, magnesium, strontium, barium, or a mixture of two or more thereof. The metal for the additional metal salt may be iron, manganese, tin, zinc, or a mixture of two or more thereof. The anion for the additional metal salt may be an inorganic anion such as a phosphate, halide, sulfate or nitrate, or an organic anion such as acetate. In various embodiments, the additional metal salt may be an alkaline earth metal salt such as an alkaline earth metal phosphate. In various embodiments, the additional metal salt may be a magnesium salt such as magnesium phosphate. In various embodiments, the additional metal salt may be an alkaline earth metal nitrate, an alkaline earth metal halide, an alkaline earth metal sulfate, an alkaline earth metal acetate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may be magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof. In various embodiments, the additional metal salt may comprise: (i) magnesium phosphate; and (ii) a magnesium nitrate, magnesium halide, magnesium sulfate, or a mixture of two or more thereof.

The additional metal salt may be selected with reference to its compatibility with other ingredients in the first slurry and/or the second slurry. Compatibility may include metal phosphates that do not precipitate, flocculate, agglomerate, react to form undesirable species, or settle out prior to application of the first slurry and/or the second slurry to the carbon-carbon composite. The phosphates may be monobasic (H₂PO₄ ⁻), dibasic (HPO₄ ⁻²), or tribasic (PO₄ ⁻³). The phosphates may be hydrated. Examples of alkaline earth metal phosphates that may be used include calcium hydrogen phosphate (calcium phosphate, dibasic), calcium phosphate tribasic octahydrate, magnesium hydrogen phosphate (magnesium phosphate, dibasic), magnesium phosphate tribasic octahydrate, strontium hydrogen phosphate (strontium phosphate, dibasic), strontium phosphate tribasic octahydrate and barium phosphate.

In various embodiments, a similar compound(s) to the additional metal salt may be used as the additional metal salt. Similar compounds include compounds that yield a similar compound to the additional metal salt in response to an outside stimulus such as, temperature, hydration, or dehydration. For example, similar compounds to alkaline earth metal phosphates may include alkaline earth metal pyrophosphates, hypophosphates, hypophosphites and orthophosphites. Similar compounds include magnesium and barium pyrophosphate, magnesium and barium orthophosphate, magnesium and barium hypophosphate, magnesium and barium hypophosphite, and magnesium and barium orthophosphite.

While not wishing to be bound by theory, it is believed that the addition of multivalent cations, such as alkaline earth metals, transition metals and nonmetallic elements such as boron, to the first slurry and/or the second slurry enhances the hydrolytic stability of the metal-phosphate network. In general, the hydrolytic stability of the metal-phosphate network increases as the metal content increases, however a change from one metallic element to another may influence oxidation inhibition to a greater extent than a variation in the metal-phosphate ratio. The solubility of the phosphate compounds may be influenced by the nature of the cation associated with the phosphate anion. For example, phosphates incorporating monovalent cations such as sodium orthophosphate or phosphoric acid (hydrogen cations) are very soluble in water while (tri)barium orthophosphate is insoluble. Phosphoric acids can be condensed to form networks but such compounds tend to remain hydrolytically unstable. Generally, it is believed that the multivalent cations link phosphate anions creating a phosphate network with reduced solubility. Another factor that may influence hydrolytic stability is the presence of —P—O—H groups in the condensed phosphate product formed from the first slurry and/or the second slurry during thermal treatment. The first slurry and/or the second slurry may be formulated to minimize concentration of these species and any subsequent hydrolytic instability. Whereas increasing the metal content may enhance the hydrolytic stability of the first slurry and/or the second slurry, it may be desirable to strike a balance between composition stability and effectiveness as an oxidation inhibitor.

In various embodiments, the additional metal salt may be present in the first slurry and/or the second slurry at a concentration in the range from about 0.5 weight percent to about 30 weight percent, and in various embodiments from about 0.5 weight percent to about 25 weight percent, and in various embodiments from about 5 weight percent to about 20 weight percent. In various embodiments, a combination of two or more additional metal salts may be present at a concentration in the range from about 10 weight percent to about 30 weight percent, and in various embodiments from about 12 weight percent to about 20 weight percent.

Method 400 may further comprise a step 250 of heating the composite structure to form a sealing layer, which may comprise phosphate glass, over the base layer. Similar to step 230, the composite structure may be heated at a temperature sufficient to adhere the sealing layer to the base layer by, for example, drying or baking the carbon-carbon composite structure at a temperature in the range from about 200° C. (392° F.) to about 1000° C. (1832° F.). In various embodiments, the composite structure is heated to a temperature in a range from about 600° C. (1112° F.) to about 1000° C. (1832° F.), or between about 200° C. (392° F.) to about 900° C. (1652° F.), or further, between about 400° C. (752° F.) to about 850° C. (1562° F.), wherein in this context only, the term “about” means plus or minus 10° C. Further, step 250 may, for example, comprise heating the composite structure for a period between about 0.5 hour and about 8 hours, where the term “about” in this context only means plus or minus 0.25 hours.

In various embodiments, step 250 may comprise heating the composite structure to a first, lower temperature (for example, about 30° C. (86° F.) to about 300° C. (572° F.)) followed by heating at a second, higher temperature (for example, about 300° C. (572° F.) to about 1000° C. (1832° F.)). Further, step 250 may be performed in an inert environment, such as under a blanket of inert or less reactive gas (e.g., nitrogen, argon, other noble gases, and the like).

TABLE 1 illustrates a variety of slurries comprising pre-slurry compositions, including phosphate glass compositions, prepared in accordance with various embodiments.

TABLE 1 Example>> A B C h-Boron nitride powder 8.25 8.25 — Graphene nanoplatelets 0.150 0.150 — Surfynol 465 surfactant 0.2 0.2 — H₂O 60.0 60.0 52.4  Ammonium dihydrogen phosphate (ADHP) — — 11.33 Poly(acrylic acid, sodium salt) — 0.50 — Cellulose — 5.0 — Glass frit 26.5 26.5 34.0  Acid Aluminum Phosphate (Al:P = 1:2.5) 5.0 5.0 —

As illustrated in TABLE 1, two first slurries for oxidation protection systems,

Slurries A and B, were prepared, such as first slurries applied in step 220 of methods 200, 300, and 400, which may provide base layers on a composite structure. All amounts listed in TABLE 1 are listed in grams. Slurry A comprises a pre-slurry composition, comprising phosphate glass composition glass frit and various additives including h-boron nitride, graphene nanoplatelets, a surfactant, and acid aluminum phosphate, in a carrier fluid (i.e., water). Slurry B comprises a pre-slurry composition, comprising phosphate glass composition glass frit and various additives including h-boron nitride, graphene nanoplatelets, a surfactant, a (sodium) salt of poly(acrylic acid) as a secondary flow modifier, and acid aluminum phosphate, and cellulose as a primary flow modifier, in a carrier fluid (i.e., water). Slurry C may be a suitable second slurry which will serve as a sealing layer when heated (such as during step 250).

With combined reference to TABLE 1 and FIG. 3, the first slurries, Slurries A and B, were applied to 50 gram carbon-carbon composite structure coupons and cured in an inert atmosphere under heat at 899° C. (1650° F.) to form base layers. After cooling, second slurry, Slurry C, was applied atop the cured base layers and the coupons were fired again in an inert atmosphere. After cooling, the coupons were subjected to isothermal oxidation testing a 760° C. (1400° F.) over a period of hours while monitoring mass loss. In FIG. 3, percent weight loss of the composite structure is depicted on the y-axis and heat exposure time 760° C. (1400° F.) is depicted on the x-axis. The composite structure with Slurry A applied to form the base layer is illustrated by data set 305, while the composite structure with Slurry B applied to form the base layer is illustrated by data set 310. As shown, after 24 hours of heat exposure, FIG. 3 shows the composite structure represented by data set 310 experienced less weight loss after 24 hours of heat exposure than the composite structure represented by data set 305. Therefore, the oxidation protection system comprising Slurry B, which comprised a primary flow modifier (cellulose) and a secondary flow modifier (poly(acrylic acid)), provides more effective oxidation protection than the oxidation protection system comprising Slurry A, which does not comprise a primary or secondary flow modifier. As discussed above, the primary flow modifier, cellulose, in Slurry B surprisingly interacts and/or bonds with the other compounds (and cellulose molecules) in Slurry B, facilitating more a uniform, continuous application of Slurry B and resulting base layer after heating (such as heating step 230 in methods 200, 300, and/or 400), than Slurry A (without cellulose). Additionally, the secondary flow modifier of Slurry B, a salt of poly(acrylic acid) (e.g., sodium salt of poly(acrylic acid)), lowers the viscosity of Slurry B, such that Slurry B may be better distributed on the composite structure via spraying. Therefore, the inclusion of a primary flow modifier (e.g., cellulose) and a secondary flow modifier (e.g., poly(acrylic acid, sodium salt)) in a first slurry (e.g., Slurry B) provides comparable or better oxidation protection than a first slurry without such components (e.g., Slurry A).

Benefits and other advantages have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C.

Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments.

Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112(f), unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 

What is claimed is:
 1. A method for coating a composite structure, comprising: forming a first slurry by combining a first pre-slurry composition comprising a first phosphate glass composition, with a primary flow modifier and a first carrier fluid, wherein the primary flow modifier comprises at least one of cellulose or calcium silicate; applying the first slurry on a surface of the composite structure to form a base layer; and heating the composite structure to a temperature sufficient to adhere the base layer to the composite structure.
 2. The method of claim 1, wherein the first pre-slurry composition further comprises a secondary flow modifier that comprises at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid).
 3. The method of claim 1, wherein the first pre-slurry composition comprises an acid aluminum phosphate, wherein a molar ratio of aluminum to phosphoric acid is between 1 to 2 and 1 to
 3. 4. The method of claim 1, further comprising: forming a second slurry by combining a second pre-slurry composition with a second carrier fluid, wherein the second pre-slurry composition comprises a second phosphate glass composition; applying the second slurry to the base layer; and heating the composite structure to a second temperature sufficient to form a sealing layer on the base layer.
 5. The method of claim 1, further comprising applying at least one of a pretreating composition or a barrier coating to the composite structure prior to applying the first slurry to the composite structure.
 6. The method of claim 1, further comprising applying a pretreating composition, wherein the pretreating composition comprises at least one of a phosphoric acid or an acid phosphate salt, and at least one of an aluminum salt or an additional metal salt, and wherein the composite structure is porous and the pretreating composition penetrates a pore of the composite structure.
 7. The method of claim 1, further comprising applying a pretreating composition, wherein the applying comprises: applying a first pretreating composition to an outer surface of the composite structure, the first pretreating composition comprising aluminum oxide and water; heating the pretreating composition; applying a second pretreating composition comprising an aluminum salt and at least one of a phosphoric acid or an acid phosphate salt on the first pretreating composition, wherein the composite structure is porous and the second pretreating composition penetrates a pore of the composite structure.
 8. The method of claim 5, wherein the barrier coating comprises at least one of a carbide, a nitride, a boron nitride, a silicon carbide, a titanium carbide, a boron carbide, a silicon oxycarbide, a molybdenum disulfide, a tungsten disulfide, or a silicon nitride.
 9. The method of claim 1, wherein the first phosphate glass composition is represented by the formula a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y_(z)+z)=1; and x<(y+y₂).
 10. The method of claim 4, wherein the second phosphate glass composition is represented by the formula a(A′₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂).
 11. The method of claim 1, wherein the composite structure is a carbon-carbon composite structure.
 12. The method of claim 1, wherein the forming the first slurry comprises mixing the first pre-slurry composition and the first carrier fluid prior to mixing the primary flow modifier into the first slurry.
 13. A slurry for coating a composite structure, comprising: a carrier fluid; a primary flow modifier comprising at least one of cellulose or calcium silicate; and a first pre-slurry composition comprising a first phosphate glass composition.
 14. The slurry of claim 13, wherein the first pre-slurry composition further comprises a secondary flow modifier that is at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid).
 15. An article, comprising: a carbon-carbon composite structure; and an oxidation protection system including a base layer disposed on an outer surface of the carbon-carbon structure, wherein the base layer comprises a first pre-slurry composition having a first phosphate glass composition, and a primary flow modifier, wherein the primary flow modifier is at least one of cellulose or calcium silicate.
 16. The article of claim 15, wherein the first pre-slurry composition of the base layer comprises a secondary flow modifier that is at least one of poly(vinyl alcohol) or a salt of poly(acrylic acid).
 17. The article of claim 15, wherein the first pre-slurry composition of the base layer comprises an acid aluminum phosphate wherein a molar ratio of aluminum to phosphoric acid is between 1 to 2 and 1 to
 3. 18. The article of claim 15, further comprising a sealing layer disposed on an outer surface of the base layer, wherein the sealing layer comprises a second pre-slurry composition comprising a second phosphate glass composition.
 19. The article of claim 18, wherein at least one of the first phosphate glass composition or the second phosphate glass composition is represented by the formula a(A′1 ₂O)_(x)(P₂O₅)_(y1)b(G_(f)O)_(y2)c(A″O)_(z): A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof; G_(f) is selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof; A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof; a is a number in the range from 1 to about 5; b is a number in the range from 0 to about 10; c is a number in the range from 0 to about 30; x is a number in the range from about 0.050 to about 0.500; y₁ is a number in the range from about 0.100 to about 0.950; y₂ is a number in the range from 0 to about 0.20; and z is a number in the range from about 0.01 to about 0.5; (x+y₁+y₂+z)=1; and x<(y₁+y₂).
 20. The article of claim 19, wherein the second pre-slurry composition comprises acid aluminum phosphate and wherein the second phosphate glass composition is substantially free of boron nitride. 